1. Field of the Invention
The present invention relates to a purple diamond suitable, for example, for ornamental purposes and a method of producing such a purple diamond.
2. Background Information
To obtain a colored diamond, it has been usual practice to irradiate a natural diamond with an electron beam and then anneal the same under a vacuum atmosphere, so as to form a color center.
A color center is formed when a nitrogen atom in a crystal and a lattice defect formed by the irradiation of an electron beam are combined with each other by the action of annealing. Further, a lattice defect alone is also effective in forming a colored diamond. Further, the type of the color center is determined by the manner in which nitrogen atoms aggregate together. The relation between color centers and colors is shown in Table 1.
TABLE 1 ______________________________________ Item Center State of Combination Color of Color Name in Color Center Center ______________________________________ H3 Center A pair of nitrogen atoms Yellow and a lattice defect H4 Center Two pairs of nitrogen Yellow atoms and a lattice defect N-V Center An isolated nitrogen atom Purple and a lattice defect GR1 Center A lattice defect Bluish green N3 Center Three nitrogen atoms Light yellow (naturally occurring) ______________________________________
For the methods of forming the color centers listed in Table 1 and their characteristics, reference is made to "Optical absorption and luminescence in diamonds, " in Reports on Progress Physics, by John Walker, Vol. 42, 1979. Further, the true colors exhibited by colored diamonds are the colors indicated in Table 1 superposed on the intrinsic colors of natural rough diamonds.
The intrinsic colors of natural rough diamonds are classified as shown in Table 2.
TABLE 2 __________________________________________________________________________ Item Classi- Configuration in which Nitrogen Exists Proportion fication Color of Isolated nitrogen 1 pair of 2 pairs of 3 nitrogen Occurring of Diamonds Rough Stone atoms nitrogen atoms nitrogen atoms atoms in Nature __________________________________________________________________________ IIa Transparent Absent Absent Absent Absent 1% colorless Ia Transparent Absent Present Present Absent 10% colorless Light Absent Present Present Few yellow Yellow Present Present Present Present 90% Brown Many Present Present Many IIb Blue Absent Absent Absent Absent approximately (containing B) 0% Ib Clear Present Absent Absent Absent 0.2% yellow __________________________________________________________________________
As is obvious from Table 2 and Table 1, those rough stones are colored effectively by making a color center indicated in Table 1 in type IIa or Ia diamonds which are transparent colorless or light yellow. These rough stones either do not have nitrogen at all or have 1 or 2 pairs of nitrogen atoms. Thus, it follows that coloration is effective for only two colors which follow, namely:
(1) Bluish green (based on the GR1 center formed solely by electron beam irradiation) PA1 (2) Yellow (based on the H3 and H4 centers formed by electron beam irradiation and annealing). PA1 (1) Does it induce a transparent sensation in the mind? PA1 (2) Is the color brilliant? PA1 (3) Does it have an ornamental value?
From the above observation, we have concluded that a diamond which exhibits purple developed by the N-V center can hardly be produced from natural rough stones and that a purple diamond which induces a transparent sensation in the mind and whose, color is brilliant cannot be produced from natural rough stones. Further, since the Ib type rough stone containing isolated type nitrogen alone for the production of the N-V center is present in as low as 0.2% of natural rough stones, it is very difficult to obtain such a rough stone itself.
Thus, we conceived the idea of using an Ib type artificial synthetic diamond which contains 100% isolated nitrogen. In addition,a method of producing the N-V center by electron beam irradiation and annealing is described by A. T. Collins, in "Journal of Physics C", Solid State Physics, 16 (pp. 2177-2181, 1983). However, to put a colored diamond to ornamental use, it should induces a transparent sensation in the mind and its color should be brilliant, a problem which has not been solved.